Moon may be younger than thought, study says

An analysis of a lunar rock raises questions about when and how the moon was formed. It may be 200 million years younger than widely believed.

The moon rises over Los Angeles City Hall. The new analysis could leave scientists… (Scott Harrison / Los Angeles…)

The moon may be 200 million years younger than widely believed, according to a new analysis of a rock brought back to Earth in 1972 by Apollo 16 astronauts. Or, if not, the moon may never have had the magma ocean that scientists think covered its surface soon after it formed.

Either way, the findings published online Wednesday by the journal Nature could send lunar scientists back to the drawing board to reconsider the moon's evolution.

The moon is thought to have formed from debris ejected into space after a Mars-sized body collided with the still-molten Earth about 4.5 billion years ago. The young moon would have been hot and blanketed by magma. But without a thick atmosphere to trap its heat, the molten rock cooled relatively quickly, while minerals that were less dense than the magma floated to the top first, forming the moon's crust. These rocks give the white highlands of the moon's near side their pale hue, and have been used to determine the point at which the moon solidified into the body we know today.

But an international team of scientists decided to use sophisticated techniques to better test a sample collected by the Apollo 16 mission — one that was considered one of the oldest moon rocks and that would, with any luck, provide an accurate age because it is relatively unscathed by meteoric impacts.

Planetary scientists can determine a rock's age by calculating how many radioactive "parent" isotopes of a particular element have decayed into "daughter" isotopes. But rather than test the radioactive decay using just one method, the researchers used three, involving the elements lead, samarium and neodymium. Because different isotopes decay at different rates, each method provided a slightly different measuring stick.

All three calculations resulted in very similar ages: an average of about 4.36 billion years, which surprised the scientists. "We all looked at one another and laughed," said lead author Lars Borg, a geochemist at Lawrence Livermore National Laboratory in Northern California.

If that is correct, it means the moon's magma ocean formed — and cooled — more recently than scientists have generally thought was the case based on evidence from meteorites containing some of the oldest minerals in the solar system. This also could mean that the great impact that separated the moon from Earth happened more recently too.

The study authors propose another, more radical, explanation: The crustal rock they analyzed, called ferroan anorthosite, is not linked to magma dynamics at all. Perhaps the moon never even had a magma ocean and the rocks were formed another way, they suggested.

"You're left with picking your poison," Borg said.

The new dates could leave scientists who model the moon's formation "scratching their heads," said Alex Halliday, an isotope geochemist at Oxford University who was not involved in the study. "It's a little bit awkward, because nobody likes to say, 'They've got their data wrong.' "

But there are less dramatic explanations, Halliday said, including the possibility that both these and previous dates are right, and the ferroan anorthosite examined in this study simply does not represent the oldest rocks on the lunar surface.

"I hope it's going to cause a real stir," said Clive Neal, a planetary geologist at the University of Notre Dame who was not involved in the study. But, he added, the researchers need much more evidence that other rocks have been inaccurately dated before they jump to radically different theories about the moon's formation.